This invention relates to electron filaments, and particularly to a line filament which is substantially uniform in cross section about its longitudinal axis.
Electron filaments are either directly heated or indirectly heated. A directly heated filament includes a refractory metal core, such as tungsten, which either functions as an electron emitter itself, or which is coated with an emissive material, such as emission carbonate. An indirectly heated filament, i.e., the emissive material is not directly heated, includes a heater element which is separated from an emissive material by a body of electrically insulating material.
Although there are numerous filaments which have been developed, none is suitable for use in a display in which a uniform line source of thermionic electrons is required. A line filament, i.e., a line source of thermionic electrons, would be particularly useful as the cathode in a flat image display device. However, for satisfactory emission uniformity, it is necessary that the line filament operate under space charge limited emission conditions. This means that the distance between the filament and the effective anode through which the electrons are collected must be accurately maintained. However, conventional filaments exhibit substantial non-uniformities in cross section and linearity along their length. These non-uniformities are primarily due to deposition techniques employed during construction. Even when put under high tensile stress, these non-uniformities and non-linearities are serious enough to cause appreciable variations in the filament to anode separation which make operation in a display device impractical. Therefore, it would be desirable to develop a line filament which is substantially uniform in cross section along its longitudinal axis.